The present invention relates generally to accelerometers, and finds particularly efficacious application in implantable cardiac pacemakers as an accelerometer for generating an electrical output signal indicative of a patient's activity.
Present day piezoceramic cantilevered beams are well understood in the area of cardiac pacing, as well as the equations which govern their characteristics. Conventional electrical and mechanical connection of the beam is typically accomplished by clamping onto the short edge of the beam to produce a cantilever configuration defining an overall beam length.
U.S. Pat. No. 4,140,132 to Dahl describes piezoceramic material in a physical activity sensor or accelerometer An elongated piezoelectric cantilevered element is disclosed as having a weighted mass on one end of the element, and as being enclosed within an implanted cardiac pacemaker.
U.S. Pat. No. 5,235,237 to Leonhardt discloses a piezoceramic bending beam accelerometer enclosed within a housing where surface mount technology is employed. One end of the packaged accelerometer is clamped down within an enclosed package.
U.S. Pat. No. 4,653,326 to Danel et al. describes an accelerometer capable of measuring a component of acceleration by means of a variable capacitance capacitor.
U.S. Pat. No. 5,031,615 to Alt discloses a pacemaker which employs an accelerometer comprising a miniaturized mechanoelectrical converter or transducer formed in a semiconductor device.
The inventions disclosed in preceding references have certain disadvantages. For example, the beam connection to the package or pacemaker shield becomes a dominant factor in determining the sensitivity of the accelerometer when employing a bonding medium of either solder or conductive epoxy. When bonding, the medium may bleed onto the beam and result in a reduced effective net length of the beam as well as attenuation of piezoceramic sensitivity. Hence, the bonding step can adversely affect the overall beam performance and contribute to manufacturing yield loss. Additionally, many bonding methods require complex and expensive packing techniques to ensure a robust design.
Some prior art pacemaker accelerometers suffer from excessive mechanical fragility to the extent that the center cantilever beams thereof break, fracture or otherwise fail when the pacemakers are dropped onto hard surfaces from heights of only a few feet or inches.